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I am going to get a second printer (Rostock Max v3) and completely upgrade/rebuild my Rostock Max V2. If I already have a Raspberry Pi 3 can I get the base V3 kit or are there more pieces I would need? I use Simplify3D for my slicer, will I be able to send the gcode from S3D to octoprint or astroprint? I am looking at the DuetWifi in the store and want to know if there is any advantage than just using the RAMBo and a Raspberry Pi. How do FSR’s compare to the accelerometer in the stock hotend?

ESD killed my RAMBo so I replaced it with a Duet Wifi. The major difference I encountered was the microstepping drivers make the motors much, much quieter. Now the hurricane fans in the Meanwell power supply are by far my noisiest part. I never used the autolevel on the RAMBo, so I can't compare the two. The Duet also eliminates the need for a Pi since it's a web server. The Pi could still be good for recording time-lapse, though?

Of all the random changes I made to the V2, the PEI sheet is by far the biggest improvement relative to cost and time. No more coatings! It's best if the bed can get to 110 for ABS, but PEI sticks great at 60. PET/PETG sticks a bit too well. (I don't know how well the V3 gets to hot bed temps, but that's why I added a 24V power supply and SSR for the V2.) Nylon doesn't stick to PEI at all.

The other (more involved) big payoff change was switching to a Zesty Nimble, which takes the bowden tube out of the picture, so there's far less work finding the best retraction length and it lets you easily print flexible filaments.

The big downside for me for the Nimble and Berd-Air is that they complicate any enclosures. So ABS printing is back to a shower curtain and a lot of paper clips.

So, Let's try to get things in order:

Do you need anything beyond the base kit for the raspberry pi mounting?
Yes, You want 4X of the board standoffs, and IIRC, you may need to buy or get a small amount of wire for wiring it up. Nothing major.

Can you send G-code from S3D to astro/octoprint or any other print host?
Yes. G-code is G-code. If it will run on a standard machine, it should be uploadable to web-based print hosts.

Are there advantages to the Duet Wifi:
Most definitely. Advantage one, higher micro-stepping capacity and amperage limits, meaning you could increase power and resolution with the right motors (at the highest microstepping levels, you will lose substaintial torque, and likely would need to increase the amperage a lot, and then need to bump up the voltage to get the speed back, but it is possible). No segmentation artifacts and a much cleaner, denser calibration capability, with no need for anything other than the built in leveling macros, meaning no fiddling with plugins or web pages. Additionally, it has a nicer PID tuning model, and supports keeping models on the board itself, or using a removable SD card to bring files to the machine. Beyond that, it's also got support for PT100 and thermocouple temperature sensors, which are both more accurate.

Do FSR's have any advantages for you?
If you're using the Duet WIFI, definitely, as they enable probing for it, and other 3rd party boards or firmware builds. A lot of us prefer them as well because they are connected to the bed, and don't need to be matched one to one with the hotend. You can also swap bed surfaces with less work, as the accelorometer doesn't like Fablam, Buildtak, or PEI very much. It's also less likely to deform the nozzle or chip the glass than ramming at a high speed would.

Now, I can give you opinions on other upgrades, but that handles the questions, I think.

Below is what I plan on getting so far. See the attachment for more info:

Rostock MAX v3 Kit
UltiBots FSR Kit
Trick Laser FSR Plate
Trick Laser Heat Spreader
E3D V6 All-Metal HotEnd
E3D PT100 Sensor
DuetWifi w/PT100 Board

Regarding stepper motors, what steppers would work the best for me? Will the stock power supply in the V3 be up to the task to handle everything?

The stock power supply in the V3 should handle it fine. It has 29A, of which ~12 go to the heated bed, and ~3.5 goes to a standard 40W hotend, leaving you close enough to 13A for everything else. Now, at 2A for each stepper motor, which is rarely run because that much torque usually isn't needed, you have 5A or 60 Watts of power for any lights, the electronics, and fans. That's plenty. As far as stepper motors, I suggest these This gives you more torque than the standard motors in theory, with twice the native resolution. You likely don't need them for the extruder, but if you want, it can't hurt.

Also, I very much suggest buying a piece of PEI, and attaching it to either the heat spreader (I went that route, since metal fabrication is easy for me, and I had multiple heat spreaders), or the glass. That way most things such as ABS stick really well with no gluestick.